Optically encoded acoustic to digital transducer

Abstract

A transducer converts an acoustic signal into a digital pulse code signal through optical encoding techniques. A pulsed light source emits a beam of light along a predetermined path. A digital optical encoder is located in the path and emits a digitally encoded light beam having regions in which light is present and regions in which there is an absence of light. A rotatable mirror which is located in the path of the encoded beam is connected to a diaphragm so that the angle of deflection of the encoded beam is a function of the acoustic signal. An array of digitally encoded photoconductive elements is located in the path of the deflected encoded beam. Threshold circuitry monitors the variation in the resistance of the photoconductive elements and produces an N bit digital pulse code representative of the magnitude and polarity of the acoustic signal during each occurrence of a pulsed light beam.

I claim: 
     
       1. An optically encoded transducer for use in a telephone handset for converting analog acoustic energy into an electrical digital pulse code comprising: a. source of optical powwer for generating a beam of light along a predetermined path,   b. means disposed in the path for encoding the beam into a preselected pattern of light having a plurality of rows of alternating regions in which light is present and in which light is absent, the areas of the regions in the respective rows being related by factors of two to define a binary weighted relationship between the rows of regions,   c. an array of thin film photoconductive elements, having a plurality of rows of spaced photoconductive elements, the areas of the elements in the respective rows being related by factors of two to define a binary weighted relationship between the rows of elements, the array being disposed in the path of encoded beam such that a row of regions impinges upon a defined row of photoconductive elements, the shapes and areas of corresponding regions and elements in rows of the array and the encoded beam being substantially equal as the encoded beam impinges the aray, the photoconductive elements of each row being electrically interconnected and generating a value of resistance which is related to the relative position of the light containing regions with respect to the photoconductive elements,   d. the number of photoconductive elements in any row except that having the greatest binary weighted value being twice the number of light containing regions in a corresponding row of the encoded beam so that the resistance of each row varies between a first level in which all elements are not exposed to light and a second level where one-half the elements are exposed to light,   e. means responsive to the acoustic energy for effecting relative motion between the array and the encoded beam in a direction parallel to the rows to vary the resistance of each row of elements between the first and second levels, and   f. means for establishing a threshold level between the first and second levels and for generating first and second digital voltages for each of the plurality of rows of photoconductive elements depending upon whether the resistance level is above or below the threshold level.   
     
     
       2. The transducer according to claim 1 wherein the source includes a pulsed light emitting diode. 
     
     
       3. The transducer according to claim 1 wherein the encoding means includes a mask disposed along the path, the mask having light transmitting and light blocking regions. 
     
     
       4. The transducer according to claim 1 wherein the photoconductive elements in each row are connected in parallel to maximize the excursion between the first and second levels of resistance. 
     
     
       5. The transducer according to claim 2 further including means for converting the output of the threshold level establishing means into serial form for transmission. 
     
     
       6. The transducer according to claim 1 wherein the resistance threshold level is 50% of the excursion between the first and second resistance levels. 
     
     
       7. The transducer according to claim 1 wherein the array is fixed and the means for effecting relative motion between the encoded beam and the array includes means for deflecting the encoded beam as a function of the acoustic energy. 
     
     
       8. The transducer according to claim 7 wherein the deflecting means includes a mirror having a reflective surface and being located in the path of the encoded beam, and means for changing the angle of incidence of the encoded beam to the reflective surface by rotating the mirror about an axis of the mirror, the photoconductive elements being located in the path of the encoded beam reflected from the mirror. 
     
     
       9. The transducer according to claim 8 wherein means for changing the angle of incidence includes a diaphragm for receiving the acoustic energy, the diaphragm having an output member connected to a first side of the mirror and a suspension wire affixed to the other side of the mirror so that the mirror may rotate about the suspension wire. 
     
     
       10. The transducer according to claim 9 wherein the nominal angle of the mirror to the encoded beam is 45°. 
     
     
       11. The transducer according to claim 9 wherein for a rotation of the mirror through an angle θ, the image of the encoded beam on the photoconductive array appears as though rotated through an angle of 2 θ. 
     
     
       12. The transducer according to claim 9, wherein the ratio of the linear dimensions of the elements to the linear dimensions of the regions on the mask is equal to the sum of the distance between the source and the mask and the optical path between the mask and the photoconductive elements divided by the distance between the source and the mask. 
     
     
       13. The transducer according to claim 12, wherein the ratio is three.